Backport CoreCLR support to v1.0.x

Update argument documentation for channel methods

Update argument documentation for request_serializer and response_deserializer
in:

- unary_unary
- unary_stream
- stream_unary
- stream_stream

Add 'node6' to the other spot where it's needed in run_tests.py

remove log statements that showing contents of messages

fail benchmarks with errors in a child rpc thread

update and add new generate_protos script in csharp examples

update nuget client to latest version on performance workers

change hint paths for protobuf to use net45 dir in csproj files

destroy byte buffer reader after use

Add Node 6 to the list of recognized compilers in run_tests.py

build.yaml changes to make certain grpc++ builds work